Method of making photocells

ABSTRACT

A method of making photocell includes forming first subassemblies each of a glass tube having secondary emissive internal surfaces and a tubular electrode and insulated inner coaxial conductor inserted at one end, sealing the elements together, applying a metal conductor layer such as copper over the opposite tube end forming second subassemblies each of a glass window disk having a cylindrical projection on one side to fit within said tube and a photo cathode layer on the end thereof, applying a second conducto layer over the one side of the window, depositing a metal sealant layer such as indium onto said second layer, processing the photo cathode, and bringing the first and second subassemblies together with the glass tube fitting over the projection at a temperature to melt the sealant and seal the elements together to form a complete photocell.

y 29, 1973 J. M. GRANT METHOD OF MAKING PHOTOCELLS 2 Sheets-Sheet 1 Original Filed March 23 1970 w m w W V w Fm Z.

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lnvenlor JOHN M- E A tlorney y 9,1913 J. M. GRANT 3,736,207

METHOD OF MAKING PHOTOCELLS Original Filed March 25, 1970 2 Sheets-Sheet 2 I rwenlor Jon/v M. ERA N Attorney United States Patent O 3,736,207 METHOD OF MAKING PHOTOCELLS John Milton Grant, 11645 Woodley Ave., Granada Hills, Calif. 91344 Original application Mar. 23, 1970, Ser. No. 21,918, new Patent No. 3,634,690. Divided and this application Apr. 14, 1971, Ser. No. 133,948

Int. Cl. B29b 27/12 US. Cl. 156294 6 Claims ABSTRACT OF THE DISCLOSURE A method of making photocells includes forming first subassemblies each of a glass tube having secondary emissive internal surfaces and a tubular electrode and insulated inner coaxial conductor inserted at one end, sealing the elements together, applying a metal conductor layer such as copper over the opposite tube end, forming second subassemblies each of a glass window disk having a cylindrical projection on one side to fit within said tube and a photocathode layer on the end thereof, applying a second conductor layer over the one side of the window, depositing a metal sealant layer such as indium onto said second layer, processing the photocathode, and bringing the first and second subassemblies together with the glass tube fitting over the projection at a temperature to melt the sealant and seal the elements together to form a complete photocell.

BACKGROUND OF THE INVENTION This application is a division of copending application Ser. No. 21,918 filed Mar. 23, 1970, now Pat. No. 3,634,690, for Photocells and Method of Making the Same. The benefit of the filing date of said copending application is, therefore, claimed for this application.

This invention relates to photoelectric devices, and more particularly, to photocells and a method of making a large group of them at one time.

In the past, photocells have been made with the use of dynode electron multipliers. These prior art photocells have been relatively large in size due to the use of the dynode multipliers. Further, prior art photocells have not been adapted to mass production techniques.

SUMMARY OF THE INVENTION In accordance with the present invention, the abovedescribed and other disadvantages of the prior art are overcome by providing a photocell with a channel-type electron multiplier. Due to the fact that such multipliers are generally made of glass, the glass can be used as part of a photocell evacuated envelope as well as part of a multiplier. The size of such a photocell may, thus, be reduced to an extraordinary degree. Further, by use of the method of the invention, the envelopes of a large number of photocells may be sealed simultaneously to a corresponding number of light inlet Windows for high production efficiency.

The above-described and other advantages of the invention will be better understood from the following description when considered in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAlVINGS In the drawings, which are to be regarded as merely illustrativef FIG. 1 is a longitudinal sectional view of a photocell constructed in accordance with the present invention;

FIG. 2 is a longitudinal sectional view of an alternative embodiment of the invention;

FIG. 3 is a longitudinal sectional view of a channeltype electron multiplier; and

FIG. 4 is a perspective view of photocell assemblies illustrating the method of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The photocell structures of the present invention are shown at 10 and 11 in FIGS. 1 and 2, respectively. In FIG. 1, an outer cylinder 12 forms the tube envelope and, at the same time, serves as the channel multiplier. The cylinder 12 can be made from various types of ceramics or glasses, but the most successful material to date has been lead-doped glass. An electrical feedthrough is sealed to one end of the cylinder 12. This feedthrough consists of an outer metal cylinder 13 which makes electrical contact to the channel walls and a second metal conductor 14 insulated from cylinder 13 by a suitable ceramic or glass dielectric 15. The subassembly of cylinder 13, conductor 14, and dielectric 15 is sealed to cylinder 12 with a conventional glass seal or other suitable material.

If cylinder 12 is made of lead glass, the subassembly of cylinder 12, cylinder 13, conductor 14 and dielectric 15 is hydrogen-fired to develop the proper surface conductivity on the channel wall, i.e., the internal cylindrical surface of cylinder 12. *If the conductivity of the cylinder 12 is determined when the cylinder is formed, as in the case of several semiconducting glasses and ceramics, this additional step is not necessary.

The open end of cylinder 12 is then evaporated with copper layer 16 or other suitable metal. The purpose of layer 16 is twofold: It insures a good electrical contact to the secondary emissive surface, particularly when the channel fields are determined by the surface conductivity; and it also provides a surface to which indium and/or indium alloys wet readily. The internal surface of cylinder 12 supports secondary emission.

A tube window 17 is provided which can be constructed from any one of a variety of common glasses by any conventional extruding, grinding, photoetching, or a number of other glass-forming techniques. Window 17 is evaporated with a copper layer 18 or other metal. A photocathode is fixed to an end projection 20 of window 17. Layer 18 makes contact to photocathode 19 and also serves as a wettable surface for indium and its alloys.

The copper-coated window 17 is heated and a layer 21 of iridium or indium alloy is applied to the copper layer 18. The subassembly of cylinder 12 with layer 16 and all the structure at the right end of cylinder 12 is mounted with layers 18, 21, and window 17 in a conventional bakeable vacuum chamber equipped with evapora tion sources and materials for forming the desired photocathode 19. The photocathode 19 is formed, and then the two subassemblies are brought together. The temperature of the subassemblies is raised to the melting point of the indium, and the two subassemblies are sealed together.

The operation of the photocell 10 is straightforward. A potential difference is applied between layer 16 and cylinder 13, the latter being positive with respect to the former. This establishes the appropriate fields along the internal surface of cylinder 12. :Electrons emitted from photocathode 19 are multiplied by repeated impacts with the secondary emissive internal surface of cylinder 12. The multiplied current is then collected at conductor electrode 14 which is maintained at a positive potential relative to cylinder 13.

A second version of a'miniature photocell 11 is shown in FIG. 2. This tube 11 differs from that in FIG. 1 in that it contains a mosaic array or plate 23 of channels 22. This is shown in FIG. 3. Also, see Electro-Optical Systems Design, November/December 1969, page 50.

Plate 23 is made of glass and has holes or channels 22 which have surfaces that will support secondary emission.

Plate 23 has evaporated, conductive electrodes 24. and 25 fixed thereto which have holes the same size as and which lie in registry with channels 22. The electrodes 24 and 25 lie in electrical contact with the secondary emissive surfaces of channels 22 at corresponding ends thereof. Plate 23 may be sealed to the internal wall of cylinder 26.

Since the channel diameter in a mosaic array can be quite small, the length of the channels can also be reduced. This permits a significant shortening of the tube compared to a single channel version. However, identical parts are indicated by prime reference numerals. Glass cylinder 26 is identical to cylinder 12 except that the former is shorter than the latter.

Plate 23 with electrodes .24 and 25 may be made by a conventional process. When cylinder 13 and window 17' are sealed as before, both are made to physically contact the respective faces of the electrodes 25 and 24. In the case of the contact between window 17' and electrode 24, a concave or recessed area may be preformed therein, if desired, to prevent field emission and damage to the photocathode 19 when the contact is made.

Both of the photocells and 11 are readily adaptable to multiple cathode processing and sealing techniques. Subassemblies of 12, 13, 14, and 16 or 26, 13', 14', 15', 16, 23, 24, and 25 are first formed and arranged in a mosaic array 27 as shown in FIG. 4. Windows 17 and 17 can be made from a single piece of glass which has been extruded, ground or photoetched.

Since the shape of the photocathode substrate is not important, projections 26 can have any cross section which is convenient. It is, however, necessary for sealing and optical transmission that the windows be polished to at least a window glass finish.

iWindows 17 and 17 are then assembled in a tray 28 as shown in FIG. 4. This tray should be semitransparent to permit optical monitoring of the photo surface while it is being formed. The tray should also be capable of providing good thermal contact to windows during the sealing process. One method of accomplishing this is to make the bottom of the tray out of glass and recess a Nichrome heater wire. Windows 17 and 17 are then evaporated with copper or other suitable metal. The copper is wiped from the cathode area, and the tray is covered with indium or an indium alloy. It is recommended that indium be applied in a vacuum to limit the amount of surface oxide formed but this is not absolutely necessary.

Mosaic array 27 and windows 17 and 17 are mounted in a bakeable vacuum chamber equipped with evaporation sources and material for cathode processing. The cathode is formed on the cylinder projections of windows 17 and 17'. Mosaic array 27 then is moved into position over the ends of windows 17 and 17'. The indium or indium alloy is then heated to its melting point, and the final seal is made by pushing the mosaic array 27 of tube envelopes into the molten indium and letting the indium cool. The tubes are then removed from the vacuum chamber and separated by cutting the indium layer between tubes.

What is claimed is:

1. A method of making photocells comprising the steps of:

forming first subassemblies each of a glass tube having secondary emissive internal surfaces, a tubular electrode inserted into said tube to extend into and out of one end thereof for connection to said surfaces, and an inner coaxial metal conductor extending within and out of said tubular electrode and insulated therefrom by an inner dielectric material, sealing said tubular electrode within said glass tube and said inner dielectric and conductor within said tubular electrode, and applying a first metal conductor layer over the opposite end of said tube to extend internally and externally;

forming second subassemblies each of a disk shaped glass window having a central cylindrical projection on one side adapted to fit within said tube and a photocathode layer on the end of said projection, applying a second metal conductor layer over the window surface on said one side and on the side walls of said cylindrical projection extending to said photocathode, depositing a metal sealant layer onto said second layer; forming the photocathode layer; and

bringing said first and second subassemblies together with said glass tube fitting over said projection at a temperature to melt the sealant and seal said first and second subassemblies together to form a complete photocell.

2. The method of claim 1 wherein said first subassem- I blies include a glass plate having electrodes on opposite sides and secondary emissive channels therethrough between said sides, said plate being sealed within said glass tube with said sides between said photocathode and tubular electrode.

3. The method of claim 1 including arranging said first subassemblies in a first array, arranging said second subassemblies in a second array in a tray with the windows down and photocathode ends extending outwardly, said second layer being applied to said array of second subassemblies, removing said second layer from said photo cathodes, depositing a metal sealant layer in said tray onto said second subassemblies array to cover said windows with the projections and photocathode ends being out of said layer, positioning said arrays of said first and second subassemblies in a vacuum chamber, forming said photocathodes, positioning said first array over said second array, heating said sealant layer to its melting point, pressing said first array over said second array into the molten layer with said glass tubes fitting over respective window projections to form seals between respective first and second subassemblies, and cooling said seals.

4. The invention as defined in claim 3, wherein said depositing step is performed by heating said sealant to a molten state and pouring it into said tray.

5. The method of claim 3 including cutting the metal sealant layer between adjacent sealed subassemblies to form a plurality of glass tube photocells.

6. The method of claim 5 wherein said metal conductor layers are of copper and said metal sealant is of indium.

References Cited UNITED STATES PATENTS 3,278,356 10/1966 Katz 156294 2,947,653 8/1960 Fohr 156-294 3,421,203 1/1969 Ullman et a1. 2502ll R X ALFRED L. LEAVITT, Primary Examiner C. WESTON, Assistant Examiner U.S. Cl. X.R. 156293 

